In rotary dobbies the main crank arm is mounted idly on the drive shaft via a cam which is interposed between the shaft and the big end of the crank arm. The cam is rigidly connected by a key to said crank arm or to said shaft and moves with intermittent motion in the form of 180.degree. rotations followed by a pause. The key is housed in a suitable radial groove in said cam and during said drive shaft pauses is urged by mechanical movement means such that one of its ends engages one of two opposing radial grooves provided in the big end of the main crank arm or one of two opposing radial grooves in the drive shaft.
In this manner each time the key rigidly connects the cam to the drive shaft, the cam is rotated to produce a rocking of the relative main crank arm, with consequent movement of the relative loom heddle frame.
Rotary dobbies therefore operate said frames in a totally positive manner, in that they move them upwards and downwards without the use of springs.
However, for moving the keys current rotary dobbies use an internal control device of negative type in the sense that the said mechanical movement means are used only to extract the key from the grooves in the drive shaft, whereas the key is inserted into said grooves in the aid of a spring acting either directly or indirectly on the key.
This arrangement gives rise to a series of drawbacks due substantially to the presence of the spring. This means that the speed of current rotary dobbies is limited to the extent that they are unable to operate on looms with a speed exceeding about 600 beats per minute. In this respect, at very high speeds of the order of 1200 beats per minute, at which modern air or water looms are currently able to operate, the movement of the key must be able to take place within a very short time. This cannot be achieved with complete reliability using springs. Springs, in addition to inducing deleterious uncontrolled vibrations in the key, have to be able to provide a key extraction force which is at least three times that which would be theoretically necessary. This is because of the need to foresee an overdimension of the spring by a factor of two as the applied force required to move the key is not known exactly mainly on account of the key friction and vibration. Such high forces create virtually insurmountable difficulties in constructing the relative operating members for extracting the keys within the limited spaces available. These are standardized at 12 mm in an axial direction.
A further drawback is the continuous wear of the edges of the key against the corresponding edges of the radial grooves in the shaft since the key, which is kept constantly pressed by the spring against the shaft, tends to enter the relative recess in the shaft as soon as it begins to face it, and before the shaft has completely stopped. In addition, when the shaft has stopped moving, the key completes its insertion into the groove not in accordance with an optimum law of motion with braking at the end of its travel, but instead with accelerated motion which terminates with sudden stoppage when the key hits the wall at the end of its travel, with consequent impact and rebound. As a result of this inability of current rotary dobbies to operate within the high speed range of air or water looms, it has been necessary for this range to use (non-rotary) ratchet-type dobbies able only to pull the heddle frames in one direction, the frames being moved in the opposite direction by large springs.
These ratchet-type dobbies, known as "negative dobbies", are able to operate at these high speeds because the large springs are able to eliminate the entire slack of the system and the consequent vibration, but against this they are able to drive the frames through only short paths of travel, and as the path lengths increase with the number of frames used it has proved necessary to limit the number of said frames and consequently to limit the patterns which can be incorporated into the fabrics produced on such looms. In this respect, as stated the presence of the large springs necessitates frame operating forces which are three times the forces theoretically required if the springs are absent, and in order to reduce the operating forces to values which can be actually provided by the negative ratchet dobby it has been necessary to limit the length of frame travel. These dobbies also have the further drawbacks of considerable overall size to the detriment of the loom dimensions, and a high cost determined mainly by the large springs, in addition to which such dobbies are very noisy and delicate.